1. Field of the Invention
The present invention relates to a transfer device, an image forming apparatus, and a method of transferring a developer to a sheet.
2. Description of the Related Art
Electrophotographic image forming apparatuses typically transfer static toner patterns formed on image carriers to sheets by applying direct-current (DC) voltages to the static toner patterns to move developers such as toner forming the static toner patterns to the sheets.
Some types of sheets such as Leathac paper or Japanese paper having low surface smoothness with their surfaces having large ridges and valleys involve a problem in that prints on the valleys are pale because it is more difficult for developers to be transferred to the valleys than to the ridges.
To address such a problem, a technique has been developed in which an alternating-current (AC) voltage is superimposed on a DC voltage used for a transfer operation to oscillate a developer, thereby increasing a transfer rate of the developer to the valleys (e.g., Japanese Patent Application Laid-open No. 2008-058585).
In such a conventional technique, however, the developer scatters, thereby causing bleeding of images because the developer is oscillated. Accordingly, sheets having high surface smoothness with their surfaces having smaller ridges and valleys preferably undergo a transfer operation using a DC voltage.
Therefore, there is a need for a transfer device that can improve image quality regardless of the surface smoothness of sheets and an image forming apparatus including the transfer device.